The heart is the earliest organ to form in the developing embryo providing the basis for a functional circulation as other organ systems develop. Emerging bioengineering and biotechnology approaches to study the formation of the mesoderm and its cellular lineages are a great opportunity to develop new insights into this complex developmental process. In particular, human pluripotent stem cells (hPSCs) provide an ideal system with which to study these questions because they are of human origin, scalable, allow for the use of advanced molecular biology tools for analysis, and provide a simplified system for studying cell fate choices in early development. During embryogenesis, cell fate decisions are coordinated by gradients of cytokines and morphogens, which allow for differentiation and organization of multiple cell types into complex tissues1.
The capacity to direct these complex fate choices is mediated by critical spatiotemporally orchestrated cues required to direct specific cell fates and cell subtypes. Well-described anterior-posterior morphogen gradients principally involving Activin/Nodal and bone morphogenic protein (BMP4) are required for developing a polarized axis during gastrulation2-5. This polarization of mesoderm gives rise to the heterogeneous cell types of the cardiovascular system including cardiomyocytes, endocardium, vascular endothelium, and the hematopoietic system that are specified by differences in VEGF and Wnt signaling Studies using hPSCs6-11 have contributed significantly to knowledge about mechanisms of human mesodermal patterning. Because of the different degrees of differentiation efficiency for the desired cell types, diverse methods have been used to select and enrich for cardiomyocytes or endothelial cells12-20.